A conventional procedure for coloring plastic materials, or to introduce additive thereto, is to tumble in a drum the natural resin with the additive such as coloring powder, or with polymer pellets or flakes containing high concentration of colorant or other additive. The blended plastic material is then transferred, from the drum to the hopper feeding the extruder's or molding machine's screw which then plasticates and incorporates the colorant or additive into the molten polymer.
Recently, in-line blending systems have been employed for adding prescribed amounts of colorants to the natural resin and for continuously feeding the compounded material directly into the plastic machine's hopper. Various disadvantages are associated with the foregoing compounding procedures. One important disadvantage is that pigment agglomerates fail to get dispersed and are most likely to be introduced with both the coloring powder and the color concentrates into the finished product resulting in loss of color value and in a product of inferior quality, lack of homogeniety, and lower mechanical properties. Another disadvantage of this method is that the additive tends to segregate from the solid resin during transfer and also while in the hopper, because of the difference in specific gravity and/or particle size existing between the polymer and the additive. This segregation of the various materials results in a non-uniform distribution of the additive in the polymer matrix, causing color and physical property variations in the end-product. All these problems can be overcome by introducing the color or other additive as a liquid composition.
Liquid carriers for the preparation of liquid colorants suitable for application in polymers, plastics, films, and fibers have been disclosed heretofore. As such for example is the system described in the Knepper U.S. Pat. No. 3,956,088. The applicability of said prior systems, however, has been substantially limited. For example, such carriers have been inadequate due to pigment settling, incompatibility with the host polymeric material resulting in exudation, insufficient heat stability causing processing problems, interference with the mechanical and physical properties of the end-product, or to a combination of two or more of these undesirable factors.
The forementioned disadvantages can be overcome by first dispersing the additive in a suitable liquid carrier and then by pumping the prescribed quantity of this liquid dispersion directly into the throat of the extruder or molding machine. To avoid "bridging" and screw slippage, which generally occur when the liquid additive is introduced at the hopper, the liquid color is preferably metered into the molten polymer. In a more preferred embodiment, the liquid additive composition is metered into a static mixing assembly which is positioned at, or contiguous to, the exit end of the extruder or molding machine and receives the polymer melt from the plasticating screw. A system of this kind is disclosed in my copending U.S. patent application Ser. No 780,028 filed on Mar. 22, 1977.
In some systems it may be advantageous in facilitating the dispersion of the additive in the plastic material to introduce an increment of the total carrier (without additive) to the polymer composition and thereafter introduce the additive in composition with the balance of the liquid carrier. This has the effect of preconditioning the polymer composition such as by partially wetting the polymer thereby making it more receptive to the dispersion of the liquid colorant and/or other additive when it is introduced with the common liquid carrier into the polymer.
To be suitable for extrusion or molding applications, a carrier must possess at least several important characteristics. These characteristics include:
Good Dispersability. Additive agglomerates, which are broken up by shear stresses during mixing, must be prevented from re-forming. This is especially apparent with pigments. By eliminating or reducing the electrical charges present in the surface of the pigment's particles, the carrier should function to prevent the reformation of clusters or particles which tend to agglomerate through static attraction. In the case of organic or inorganic color pigments, small particle size results in greater color strength, more uniform distribution, improved physical properties, and better quality of the finished product. The latter two properties are particularly significant in fiber and film extrusion where pigment agglomerates cause yarn breakage and are clearly visible in thin-colored films.
High Polymer Loading. A carrier capable of retaining high concentrations, e.g. as high as 70-80%, should be possible for economical and processing reasons.
High Stability. The carrier preferably should withstand temperatures as high as 300.degree. C. for short periods to be suitable for processing with a broad range of commercial resins.
Universal Application. For practical general usage, the carrier should be compatible with most commercial thermoplastic resins. This is an important factor in reducing inventory cost in molding and extrusion applications.
Avoidance of Detrimental Effect on Product Properties. To be avoided are adverse effects on mechanical properties, heat deflection temperature, crocking, heat sealability, printability, etc. The properties of heat sealability and printability are especially important in film applications.
Avoidance of Odor and Toxicity. This is important particularly in food packaging end-use where off-taste and extraction limits of additives in contact with food are often critical and toxicity must be avoided.
Although principally employed for the preparation of liquid colorants, the carrier should also be suitable for the production of multi-functional dispersions that combine colorant with other desirable additives. These additives may be UV absorbers, antioxidants, antistats, flame retardants, blowing and slip agents, or any combinations of two or more of these ingredients. In this way, a single system can impart multiple functional properties to the finished products at a substantial overall saving in manufacturing cost. Hence, unless specifically excluded, when the term colorant is employed for the purpose of describing the carrier system of this invention, it contemplates also additives in general.